It is common to provide a specified resistance at the termination of a transmission line in order to reduce reflections at the sending end or the receiving end of the transmission line. For example, termination resistors are used to eliminate reflections between buffers which interface with communication lines such as Sonet/ATM lines, at STS3 (155 Mbytes/sec) or STS12 (622 Mbytes/sec) transmission rates. Such resistive terminations have also heretofore been used to reduce reflections on other types of high performance electronic circuits such as gallium arsenide circuits, radar and other synchronous circuitry.
In the past, the value of the fixed resistance attached to either the sending or receiving end of a transmission line has been chosen to add to the output impedance of the associated driving gate, such that the sum of the fixed resistor and the driving gate output impedance is essentially equal to the characteristic impedance of the transmission line. However, the value of the fixed resistance is normally chosen based upon the average distribution of values for the driving gate resistance and the characteristic impedance of the line, both of which can vary greatly. Reflections and ringing on the lines thus commonly occur, which reduces noise margin. The value of the output impedance of the driving gate, for example, can vary with various DC currents which flow through the output driving transistor. Further, often the output impedance for a logic one or zero are not symmetrical, and circuit designers must rely upon the geometric mean of the output impedance of the driving gate.
This is particularly the case for BiCMOS circuits which provide TTL interface. The logic one output impedance is usually twice as high as the CMOS interface circuits in BiCMOS, as an additional diode drop is often used to obtain the proper TTL output level. Further, present day manufacturing capabilities for controlled impedance strip lines, for instance, may have a tolerance as high as .+-.20%, thus making it difficult to determine the desired resistance for the termination resistance. Moreover, loading resulting from elements such as connectors, printed circuit board vias, test pads, bond wires, integrated circuit fanout patterns and package parasitics are also variable, which again presents difficulties when attempting to choose the desired impedance for the termination resistor.
A need has thus arisen for an adaptive termination for transmission lines for use at either the sending or receiving end of the transmission line, such that changes in impedance of the transmission line are automatically compensated for, and wherein manufacturing tolerances and the like may be automatically compensated for. Specifically, a need has arisen for a technique to automatically match the overall interconnection system characteristic impedance of a transmission circuit to reduce or eliminate reflection at either the sending or receiving end of the transmission line.